Targeted gene expression is one of the most difficult and important goals in the development effective therapies for a variety of disorders, including, for example, cell proliferative disorders such as cancer or biological stress resulting from glucose starvation in diseases such as diabetes. Two strategies for specific expression include: 1) targetable entry; and 2) tissue or cell type specific gene expression. Targetable entry involves vector engineering to change vector binding tropism thus allowing cell type specific transduction. Tissue or cell specific expression relies on restricting expression of the delivered gene exclusively to a particular type of tissue, such as a tumor.
Successful application of any method for targeting a specific tissue or cell for expression of a particular molecule (e.g., protein or nucleic acid) requires maximization of expression of the molecule in the targeted environment. The most common promoter used to drive expression of a foreign gene has been a constitutive, general-purpose viral promoter such as the MuLV LTR. These promoters, while effective in vitro, often fail to express the sequences under their control within a biologically stressed environment (Palmer et al., Proc. Natl. Acad. Sci. USA, 88:1330, 1991; Gazit et al., Cancer Res., 55:1660, 1995). These data suggest that the MuLV promoter and other constitutive or cellular promoters are not optimal for expressing a nucleic acid sequence within, for example, a fast growing solid tumor devoid of nutrients due to insufficient blood supply. Further, even if a viral promoter escapes genomic silencing, the expression pattern of the foreign gene will be constitutive in normal as well as tumor cells. Such unregulated expression could be highly problematic in gene therapy methods.
To circumvent these difficulties, stress-responsive promoters provide an attractive means for tissue-specific expression of a therapeutic agent. For example, most fast growing tumors have a heterogeneous distribution of blood supply; by having a high interstitial and a low intravascular pressure, a decrease in nutrient supply results, leading to necrosis in the center of the tumor. Glucose deprivation, calcium deprivation, chronic anoxia and low pH known to persist in poorly vascularized solid tumors induce a class of stress proteins referred to as the glucose-regulated proteins (GRPs) (Gazit et al., Cancer Res., 55:1660, 1995; Koong et al., Int. J. Radiat. Oncol. Biol. Phys., 28:661, 1994) including the grp78 gene. A rat grp78 promoter has been used as a potent internal promoter in a retroviral vector to drive expression of the neomycin phosphotransferase (neo) reporter gene in a murine fibrosarcoma model system (Gazit et al., Cancer Res., 55:1660, 1995). Such a promoter provides an attractive means for specifically expressing a therapeutic agent in a biologically stressed tissue using currently available methods in gene therapy.
There are several strategies that have been developed to accomplish gene therapy for the treatment of disorders that give rise to a biologically stressed cellular environment, such as cancer or diabetes, for example. Within these strategies, there is a need for controlled, sustained, site-specific expression of a therapeutic agent such that surrounding healthy tissue remains unaffected by the effects of the therapeutic agent.